Powder coating compositions have become increasingly important because they give off very little or no volatile material to the environment when cured. Typically, any such emissions are limited to by-products of the curing reaction, such as blocking agents or volatile condensation products. Powder coatings have found use as both decorative coatings and protective coatings.
Clearcoat-basecoat composite coatings are widely used in the coatings art and are notable for desirable gloss, depth of color, distinctness of image and/or special metallic effects. Composite systems are particularly utilized by the automotive industry to achieve a mirror-like, glossy finish with a high depth of image.
It is important for topcoats, including the clearcoat-basecoat composite coatings, to provide the desired color. When electrocoat primers are used, amine from the electrocoat primer layer may migrate during thermal cure of later applied coating layers causing undesirable yellowing in those layers. The migration and yellowing increase with longer cure times or higher cure temperatures, thus making shorter cure times and lower cure temperatures desirable for preventing or minimizing yellowing of the later applied coating layers. Yellowing of the later applied coating layers may also be caused by volatilization of amino blocking groups, e.g. amino blocking groups of an isocyanate, or by volatilization of salting amines from waterborne compositions of a lower layer, e.g. salting amines of waterborne basecoat compositions that may cause yellowing of a clearcoat applied over the basecoat composition.
Ruth et al., U.S. Pat. No. 6,140,430 describes an epoxy powder coating containing a non-crystalline epoxy resin, a small amount of a crystalline epoxy agent, a curing agent reactive with epoxy groups, and a catalyst. The crystalline epoxy resin has a melting point preferably greater than 90° C. It is included in the powder coating in an amount sufficient to reduce the coating viscosity during curing so that air entrapped in the powder particles, which would otherwise be entrapped as bubbles and causing haze in the coating, can escape.
Clark et al., U.S. Pat. No. 5,552,487 describes a thermoset powder coating composition including a crystalline compound with a melting point between 30° C. and 150° C. as a flow control and leveling agent. The patent provides as examples of the crystalline compound trimethylolpropane, neopentyl glycol, 2,5-dimethyl-2,5-hexanediol, 2,4,6-triphenylphenol, 2,2′-biphenol, 9-phenanthrol, and para-cresol.
Carbamate- and urea-functional materials have recently been widely used in automotive coatings to achieve the durability of polyurethane coatings while avoiding use of expensive, blocked polyisocyanates that require a relatively high curing temperature or use of unblocked polyisocyanates that are water-sensitive and must be mixed into the coating only prior to application. Harris et al., U.S. Pat. No. 6,150,465 discloses a slurry coating composition that includes solid particles containing a carbamate- or urea-functional compound dispersed in a liquid phase including a crosslinking component, such as a melamine formaldehyde resin. The carbamate- or urea-functional compound of the solid particles has a glass transition temperature greater than 40° C. While melting points are sharp, glass transitions such as for the Harris carbamate- or urea-functional compounds are diffuse, occurring over a temperature range. Ambrose et al., U.S. Pat. No. 6,316,109, discusses the problem of “gassing” that occurs as a result of vaporization of the alcohol generated in thermal cure with aminoplast crosslinkers.